Mold with a function ring

ABSTRACT

The invention relates to a hot-top mold for a strand casting apparatus, consisting of a hot-top, which lies on the upper side of a parting agent distributor and presses it with its underside against the surface of a mold, an overhang being formed on the radially inner surface of the hot-top, which protrudes beyond the parting agent distributor in the direction of take off of the strand, the hot-top being centered and held by an outer ring, which is releasably fastened to the mold. The mold surrounds a function ring toward the bearing surface of the mold which, together with the parting agent distributor, forms function surfaces with adjustable roughness on the surfaces. In the parting agent distributor, radial channels are formed at the upper side and the underside, the channel cross sections at the upper side and the underside being in a ratio of 1:3 to 1:5 to one another.

BACKGROUND OF THE INVENTION

[0001] This invention relates to an apparatus for strand casting ofmolten metal, and more particularly, to a hot-top mold for improving thesurface quality of hard-to-cast alloys.

[0002] Strand casting, also known as continuous casting, involves aprocess in which molten metal is introduced into a mold. During itsresidence time in the mold, the metal solidifies in contact with thewall of the mold and can be drawn downward via a movable bottom portionof the mold. A reserve of molten metal is positioned above the mold. Thereserve is also referred to as a feeder head or a “hot-top”. A partingagent, or release agent, is applied to the surface of the continuouscasting being formed, such that direct contact with the surface of themold is avoided, thereby facilitating easy removal of the casting.Parting agents may include mixtures of oils and gases. It isparticularly desirable if the oil-gas mixture is first formed close tothe mold.

[0003] Hot-top molds of the kind described above are well known in theart. European Pat. No. EP 0566 865 to VAW discloses a hot-top mold wherea parting agent, by way of a parting agent distributor, reaches thesurface of the cast strand. Two different parting agents such as oil andgas, may be fed separately or as a mixture.

[0004] Certain hard-to-cast alloys, such as aluminum alloys containinglead, zinc, tin and copper, pose casting problems that can result inpoor surface quality. Recently, such alloys have been gaining importancein the production of special alloys and machining alloy stock, which areto be used at a high cutting speed.

[0005] Another problem is the precise control of the gas pressure, whichdetermines whether the parting agent reaches the entire surface of themetal strand. Pressure fluctuations can result in surface flaws, andpressure that is too high may pose a risk that gas might escape throughthe molten metal.

[0006] U.S. Pat. No. 4,732,209 to Pechiney attempts to solve thisproblem by using a graphite ring on the inside of the mold. The porosityof the graphite ring is established so that a gas under pressure isforced from the outside through the open pores of the graphite materialto the inside of the mold and thereby acts as a parting agent betweenthe surface of the forming metal strand and the mold surface. However,while this solution addresses the problem of controlling gas pressure,this solution cannot be applied to hard-to-cast alloys.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a hot-top mold that permitsdifferent types of alloys, in particular hard-to-cast alloys, to be madewith an improved surface quality.

[0008] The present invention comprises a hot-top ring, which lies on topof a parting agent distributor and presses it against the surface of amold. An overhang on the inner surface of the hot-top ring extendsbeyond the parting agent distributor in the downward direction of themovement of the strand and forms an annular gap with the running surfaceof the mold. A function ring is position on the inner surface of themold and forms a function surface with the parting agent distributorthrough which gaseous and liquid parting agents are delivered.

[0009] The hot top can be disassembled for use with different alloys. Agaseous parting agent and a liquid (e.g. oil) are introduced into theparting agent distributor. The parting agent distributor and functionring may produce a foam layer to be used as a parting agent whichpermits improved surface quality, particularly for hard-to-cast alloys.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an exploded view of a hot-top mold, in accordance with apreferred embodiment of the invention; and

[0011]FIG. 2 is a representation of a hot-top mold used in strandcasting, in accordance with the preferred embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0012] As shown in FIG. 1, a hot-top mold 1 comprises an upper ring 1 aand a lower ring 1 b. Rings 1 a, 1 b are secured within the radiallyinside surface of an outer ring 4 which is releasably fastened to a mold3. Mold 3 surrounds at its inner surface a function ring 5, whose upperend forms a function surface with a parting agent distributor 2. Aclamping ring 6 secures hot-top rings 1 a, 1 b, parting agentdistributor 2, mold 3, and outer ring 4. An activator ring 7 liesbetween outer ring 4 and the top section 2 a of parting agentdistributor 2. A bottom part 8 and pressure plate 9 compete the lowerportion of mold 3.

[0013] Rings 1 a, 1 b of hot-top mold 1 are easy to assemble anddisassemble, facilitating the quick exchange of different parting agentdistributors and function rings according to the desired type of alloy.Loosening clamping ring 6 and outer ring 4 permits replacement of theparting agent distributor 2 and function ring 5.

[0014] Parting agent distributor 2 comprises a plate provided on bothsides with radial channels. The cross sections of the channels may be ina ratio of 1:3 to 1:5 between top and bottom sides. This ratio isimportant for controlling the composition of the parting agent mixture.By matching the surface and channel cross sections, foam production iseasily possible. Foam is especially effective as a highly viscousparting agent and permits an improved surface quality, particularly forhard-to-cast alloys.

[0015] Function ring 5 includes surfaces facing the parting agentdistributor 2 and the mold surface. Function ring 5 may comprise copperor copper alloys. It may also comprise ceramic, composite materials orgraphite. The function ring surfaces form walls with a defined roughnessfor the channels formed by the parting agent distributor 2. It isespecially desirable if the function surface forms one wall face for theliquid-bearing channel. With a precisely adjusted surface roughness andtemperature of the function surfaces, a defined change in the viscosityof the fluid parting agent can be achieved for producing a stable foamlayer.

[0016] It is advantageous if a defined, tight porosity of function ring5 and a specific density is chosen within narrow limits, such as aclosed porosity of 0-20% and a density of 1.5-10 g/cc. Additionalimprovements in the stability of the parting agent foam can be achievedby cooling the function ring 5 in order to keep the viscosity propertieson the function surfaces constant. The construction and manner ofoperation of the function surfaces and the cooling is further explainedbelow.

[0017] The top surface 2 a of parting agent distributor 2 is usedpreferably to form gas-carrying passages, and bottom surface 2 b ofparting agent distributor 2 is used for a liquid parting agent. Thepassages incorporated into the upperside 2 a are connected to apressurized gaseous medium, while the passages on the underside 2 b areconnected to a pressurized liquid reservoir. The passages must be madewith very great precision, which can be achieved by laser machining, achemical etching method, or other techniques. The cross-sectional shapeof the radial passages is important for the air content in the partingagent foam, and the surface area ratio of the passages is important forthe formation of a parting agent foam distributed uniformly over thecircumference. It is preferred that the lower radial passages beconfigured in the manner of a diffuser, with an approximately squarecross section being given to the radially outward lying passage carryingparting agent distributor 2, and also a rectangular cross section with asurface ratio of at least 1:2 formed on the radially inward lying exitside. Under these circumstances the parting agent is formed withespecially fine cells and hence it is strong, thereby reducing theliquid component of the parting agent.

[0018] Function ring 5 is preferably cooled by arranging coolingpassages 15 (See FIG. 2), in mold 3, which extend into the areaunderneath parting agent distributor 2 and function ring 5. In thisprimary cooling zone, a temperature is established for the optimumaction of the parting agent. An adjoining secondary cooling zone permitsa rapid removal of heat in the annular gap 14 through a foam layer 16,since the cooling passages here run in the direction of the descent ofthe strand and lead into a slot nozzle. Here, the pressure of thecoolant is lowered, so that the coolant is in contact with the aluminumstrand, thereby permitting heat dissipation. Lower ring 1 b has athermal conductivity of approximately 1.5 to 2.0 times greater than thatof upper ring 1 a.

[0019] The formation of certain function surfaces is critical to theformation of a stable parting agent foam. The function surfaces aresituated above and below the parting agent distributor as well as on theinside of the function ring 5. The latter is in direct contact with thepassages of the parting agent distributor 3 and cooperates in theformation of the foam.

[0020] As an innovative and supplementing feature of the functionsurfaces, activator ring 7, which can be made of various materials, liesbetween outer ring 4 and the top 2 a of the parting agent distributor 2.It thus covers the top of the passages made in parting agent distributor2. Its roughness values differ from those of the hot-top ring 1 b andcan be adjusted to the particular requirements of the parting agent. Atthe same time, the thermal gradient of the parting agent distributor 2underneath the hot-top 1 b can thereby be controlled.

[0021] Mold 3 is supplemented by an outside ring 4 and a function ring5. By means of a clamping ring 6, the system parts are assembledtogether with the inclusion of an activator ring 7. The mold 3 iscompleted by a bottom part 8 and a pressure plate 9.

[0022]FIG. 2 illustrates the basic construction of the hot-top mold. Anannular gap 11 exists underneath the overhang of hot-top ring 1 b, theside walls of annular gap 11 being formed by the hot-top overhang and bythe inside radius of the parting agent distributor 2 and by the functionring 5. Upon the introduction of gaseous and liquid parting agentswithin the volumetric ratio of the invention, a stable parting agentfoam 12 is formed, which develops as a continuous layer of foam betweenthe mold 3 and the metal strand 10.

[0023] Initially, the viscosity of the parting agent is controlled bythe surface roughness in the gas region as well as in the liquid region.The viscosity of the parting agent is the essential factor in theformation of the foam. Furthermore, the pressure and rate of flow of thedelivered gaseous and liquid medium can be controlled, so that thecomposition of the parting agent foam can be regulated within widelimits. In turn, a controlled heat removal can be performed, or else aninsulating effect can be produced with the parting agent. This isparticularly advantageous for hard-to-cast alloys.

We claim:
 1. A hot-top mold for a strand casting apparatus comprising amold (3), a parting agent distributor (2), a hot-top ring (1 a, 1 b)lying on the top (2 a) of the parting agent distributor (2) and pressingits bottom (2 b) against the top surface (3 a) of the mold (3), anoverhang being formed at the radially interior surface of the hot-topring (1 a, 1 b) protruding beyond the parting agent distributor (2) inthe withdrawal direction of the strand and forming an annular gap (11)with the mold surface, an outer ring (4) releasably fastened to the mold(3), for centering and holding the hot-top ring, a function ring (5)surrounded by the mold positioned toward the inner surface of the mold,the function ring and parting agent distributor (2) forming functionsurfaces with adjustable roughnesses on the surfaces, and radialpassages formed on the top and bottom surfaces (2 a, 2 b) of the partingagent distributor, the passage cross sections between the upper andbottom surfaces being in a ratio of 1:3 to 1:5.
 2. The hot-top mold ofclaim 1, wherein the function ring (5) includes copper or copper alloys.3. The hot-top mold of claim 1, wherein the function ring (5) includesceramic or composite materials.
 4. The hot-top mold of claim 1, whereinthe function ring (5) includes graphite material.
 5. The hot-top mold ofclaim 1, wherein a clamping ring (6) is disposed above the hot-top (1 a,1 b), by which the hot-top (1 a, 1 b), the parting agent distributor(3), the function ring (5) and the mold (3) are clamped.
 6. The hot-topmold of claim 5, wherein the function ring (5) has a closed porosity of0-20% and a density of 1.5-10 g/cc.
 7. The hot-top mold of claim 6,wherein the radial channels, incorporated into the upper side 2 a of theparting agent distributor (2), are connected to a pressurized gaseousmedium and that radial passages are formed at the underside (2 b) of theparting agent distributor (2) and are connected to a pressurized liquidreservoir.
 8. The hot-top mold of claim 7, wherein the lower radialpassages of the parting agent distributor are configured in the mannerof a diffuser, an approximately square cross section being given to theradially outward lying passage carrying the parting agent distributor(2) and a rectangular cross section with a surface ratio of at least 1:2being formed on the radially inward-lying exit side.
 9. The hot-top moldof claim 8, wherein cooling passages (15) are disposed in the mold (3),which reach into the area underneath the parting agent distributor (2)and the function ring (5), heat removal being performed in the annulargap (14) through a foam layer (16) produced by the gas and liquid mediumand issuing in the descending direction of the strand.
 10. The hot-topmold of claim 9, wherein the bottom part of the thermal conductivity ofthe hot-top (1 b) is 1.5 to 2.0 times greater than that of the upperpart of the hot-top (1 a).
 11. The hot-top mold of claim 10, wherein theupper part (2 a) of the parting agent distributor (2) is covered by anactivator ring (7).